Since 1982, we have studied the behavior and ecology of one group of muriquis inhabiting an 800 hectare forest in Minas Gerais, Brazil. This is the only ongoing long-term field study of this highly endangered primate. We have described the muriqui's unusual egalitarian society and pattern of female-biased dispersal and male philopatry while documenting the effects of seasonal changes in food availability on diet. Demographic records of recognized individuals have yielded the only data on basic life history variables, including age at first reproduction and interbirth intervals for this species. Correlations between reproductive events and ecological variables indicate pronounced reproductive seasonality, suggesting that components of muriqui diets may regulate their fertility. Dietary components may also link to the regulation of intestinal parasite infections. Brazilian and American collaborators are identifying muriqui parasites and analyzing plant foods for anthelminthic and phytosteroid activity. Long-term demographic and ecological data have provided the basis for a population viability analysis and recommendations for the conservation of this endangered primate. New collaborations are exploring the comparative behavioral ecology of the other sympatric primates at this site to understand the effects of life histories on reproduction and population dynamics. RESULTS Key accomplishments included insights into muriqui mating preferences and the inte-gration of muriqui behavioral ecology and conservation biology. Analyses of female sexual behavior and fecal steroid assays also provided key insights into the reproductive ecology of female muriquis. OBJECTIVE: To understand the interrelationships among behavior, ecology, and reproduction in wild muriquis (Brachyteles arachnoides) in a seasonal Atlantic forest fragment. FUTURE DIRECTIONS To continue to monitor the effects of demographic changes on the behavioral and reproductive ecology of this muriqui population. KEY WORDS reproduction, parasites, fertility 1997